In the manufacture of glass or the like heat fuseable vitreous materials (herein included in the term "glass" for the convenience of reference) conventionally there is used a furnace comprising a furnace chamber for containing a body of molten glass, heating means for heating the body of molten glass, and outlet means through which glass is conducted from the chamber. Conveniently, the body of molten glass is heated electrically, using Joule effect heating, said heating means thus comprising electrodes extending into the furnace chamber. Conventionally, the furnace is open-topped, thus comprising an opening through which solid state glass batch material may be delivered into the furnace chamber.
In the use of such a furnace, batch material is delivered through the opening, being deposited on the top of the body of molten glass in the furnace chamber, thus causing the whole of the upper surface of the body to be covered by a relatively cool layer of batch material to a generally uniform thickness. The lower surface of this layer continuously melts away to join the body of molten glass in the chamber, whilst molten glass is withdrawn from outlet means, advantageously from a lower portion of the chamber, whilst fresh batch material is continually spread over the upper surface to replace the melted material.
It is in practice desirable to retain the thickness of the layer of batch material substantially constant, or within upper or lower limits, which depend upon the chemical and physical nature of the mixture of raw materials included in the batch material, the temperature of the body of molten glass, and the specific rate of melting of the lower surface of the layer. Typically, an optimum thickness may be 14 inches, or may (with a different furnace design) be as little as 3 inches.
Should the layer of batch material become either too thin or too thick, it becomes difficult to operate the furnace efficiently. Since in the production of a particular glass, only very limited adjustment of the chemical and physical nature of the raw materials is feasible, and since also the production of glass of satisfactory quality depends upon the maintenance of a substantially constant temperature of the body of molten glass, the need to maintain the layer of batch material imposes limitations on the rate of removal of glass from the furnace. Thus, with a particular type of glass being produced, and with a specific furnace, the ratio of the highest and lowest rates of removal which can be tolerated (known as the production range) will be approximately 2:1.
In many glass manufacturing operations, even when producing only a single glass composition, a production range greater than 2:1 is greatly desired. In addition, some furnaces are required to melt two or more types of glass each having different characteristics with respect to the formation of the layer of batch material, which further affects the flexibility of a specific furnace.
It is one of the various objects of this invention to provide a furnace in which the production range may be increased without unduly affecting the quality of the glass manufactured.
It is another of the various objects of this invention to provide a method of melting glass, by the performance of which the rate of production of glass may be adjusted as desired.